This invention relates to a protective circuit for series connected semiconductor devices.
This invention is for an improvement on the general circuit arrangements shown in Canadian Pat. No. 959,120-Harnden, Jr., issued Dec. 10, 1974 to General Electric Company. In that patent there is described a circuit with two semiconductor devices, such as thyristors, connected in series across a voltage source, and a metal oxide varistor connected in parallel with each of the semiconductor devices. The metal oxide varistors have a nonlinear resistance characteristic that is pronounced. The varistors have a high resistance at voltages below a predetermined value and a relatively low resistance to high current conduction occurring at voltages above the predetermined value. It thereby protects the semiconductor devices against transient high voltage peaks.
It has been found that the above arrangement may not provide adequate protection under all circumstances. For example, a large external transient may require a varistor size that is quite large to provide protection. It may require one or two columns of varistors which cannot be designed to be easily integrated with the thyristor switch due to the physical size. Therefore a compromise may be necessary having in mind the likelihood of a sufficiently large external transient occurring and the size and space limitations.
There is another situation where the prior art protection could be inadequate depending on the design. If we consider a thyristor switch in parallel with a varistor in a circuit across a power source, there will normally be inductance in the circuit external to the parallel thyristor switch and varistor. If an external transient surge causes a transient current of say 1000 amperes to flow through the varistor, a voltage will be developed across the varistor and this will be directly across the thyristor switch. This voltage might be perhaps as high as 100 KV (kilo-volts). Now if the thyristor switch should be gated on at this time, perhaps by a misfire or incorrect operation or for any reason, the thyristor may be damaged. The instantaneous voltage across the low thyristor impedance would cause excessive current and excessive rate of change of current.
It is therefore an object of the present invention to provide improved protection for a circuit having series connected thyristors.
It is another object of the invention, to provide improved protection for series connected thyristors in a power system with a metal oxide varistor connected in parallel with each thyristor, by providing a metal oxide varistor connected in parallel with all the series connected varistors.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.